ds2155gt-r Maxim Integrated Products, Inc., ds2155gt-r Datasheet - Page 109

• Manufacturer Part Number: ds2155gt-r
• Description: Ds2155 T1/e1/j1 Single-chip Transceiver
• Manufacturer: Maxim Integrated Products, Inc.

20.
The DS2155 can implement the G.706 CRC-4 recalculation at intermediate path points. When this mode
is enabled, the data stream presented at TSER already has the FAS/NFAS, CRC multiframe alignment
word, and CRC-4 checksum in time slot 0. The user can modify the Sa bit positions. This change in data
content is used to modify the CRC-4 checksum. This modification, however, does not corrupt any error
information the original CRC-4 checksum may contain. In this mode of operation, TSYNC must be
configured to multiframe mode. The data at TSER must be aligned to the TSYNC signal. If TSYNC is an
input, then the user must assert TSYNC aligned at the beginning of the multiframe relative to TSER. If
TSYNC is an output, the user must multiframe-align the data presented to TSER.
Figure 20-1. CRC-4 Recalculate Method
G.706 INTERMEDIATE CRC-4 UPDATING (E1 MODE ONLY)
TPOSO/TNEGO
INSERT
NEW CRC-4
CODE
EXTRACT
OLD CRC-4
CODE
+
CRC-4
CALCULATOR
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XOR
NEW Sa BIT
DATA
MODIFY
Sa BIT
POSITIONS
TSER